Transcript Document

A Survey of Local Group Galaxies
Currently Forming Stars
Phil Massey
Lowell Observatory
April 14, 2003
The Team:
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Paul Hodge, Univ. of Washington
Shadrian Holmes, Univ. of Texas
George Jacoby, WIYN
Nichole King, Lowell Observatory
Phil Massey (PI), Lowell Observatory
Knut Olsen, CTIO/NOAO
Abi Saha, KPNO/NOAO
Chris Smith, CTIO/NOAO
Overview
We are imaging all of the galaxies of the
Local Group that are currently forming stars
– broad-band (UBVRI)
– narrow-band (H, [OIII], [SII])
with the KPNO and CTIO 4-m telescopes
and Mosaic CCD cameras.
Motivation: Our Science
The galaxies of the Local Group serve as our
laboratories for studying star formation and stellar
evolution as a function of metallicity, Z. (Z varies
by a factor of 17 from WLM to M31.)
Why should the metallicity
matter?
• Star Formation:
– Lower metallicity gas should have a lower
cooling rate, and hence higher temperatures
larger Jeans’ mass, leading to a top-heavy
IMF (Larson 1998).
But over the limited metallicity range (3x)
SMCLMCMW this effect isn’t seen!
IMF Slope in OB Associations
From Massey (2003)
Z=0.004
Z=0.008
Z=0.018
IMF
Variations that are seen in the IMF slope are
statistical, not physical (Massey 1998,
Kroupa 2001)
But what would happen if we extended this to
one-tenth solar (WLM) to 2x solar (M31)???
The answer is important for understanding the
integrated properties of galaxies at large
look-back times.
Star Formation/metallicity (cont)
• Some expect that the upper mass limit will vary as
a function of metallicity
– True only if radiation pressure acting on grains is the
limiting factor in determining the mass of the highest
mass star that can form.
• So far we find that the “upper mass limits” are
purely statistical, and not physical. What ever it is
that limits the ultimate mass of a star we have yet
to encounter it in nature (cf. Massey & Hunter
1998 ApJ 493, 180).
Why should the metallicity
matter ? (continued)
• Massive Star Atmospheres and Evolution
– Stellar winds are driven by radiation pressure
through highly ionized metal lines. Mass-loss
rates will depend upon Z, where   0.5-1.0
– This mass-loss has a profound effect on the
evolution of high-mass stars.
log [Number RSGs/WRs]
Relative number of red supergiants
(RSGs) and Wolf-Rayet stars (W-Rs)
From Massey
2003, ARAA
41 (in press)
log (O/H) + 12
log [Number RSGs/WRs]
Relative number of red supergiants
(RSGs) and Wolf-Rayet stars (W-Rs)
From Massey
2003, ARAA
41 (in press)
log (O/H) + 12
Need good observational
database
• New generation of high mass evolutionary
models are becoming available, which
include the important effects of rotation
(mixing introduced by meridional
circulation and shear instabilities).
• Need solid observational database to help
“guide” the theorists.
Our Science (continued)
Along the way we’ll find:
• The most massive supergiants.
• Luminous Blue Variables and other
luminous stars with H emission.
• Star formation rates for massive stars.
• Distribution and numbers of evolved
massive stars (RSGs, WRs).
• HII regions, SNRs, PNe, and the extent of
the diffuse emission.
Your Science
This survey will provide the source list (“finding
charts”) for spectroscopy with 8-10-m telescopes
for decades to come. Our data products include:
• “Stacked” images (UBVRI, H, [OIII], [SII])
• Individual dithered images (suitable for
photometry).
• Calibration
• Catalog of UBVRI photometry of roughly 300
million stars
What We’re Doing: The Sample
M31 (10 fields)
M33 (3 fields)
IC 10
NGC 6822
WLM
Pegasus Dwarf
Phoenix
IC 1613
Sextans A
Sextans B
How Are We Doing?
M31 (10 fields)
M33 (3 fields)
 IC 10
 NGC 6822
 WLM
Pegasus Dwarf
 Phoenix
 IC 1613
 Sextans A
 Sextans B

What We’re Doing (continued)
Aiming for a S/N of 3 at U=B=V=R=I=25,
in 1” seeing.
Also imaging in H, [OIII], [SII]
Each field 5 ditherings, then stacked.
Hasn’t All This Been Done
Before?
Yes, but not with our depth, area, photometric accuracy and
resolution!
Photographic plates had the area coverage and (usually) the
resolution*, but neither the photometric accuracy nor
depth.
CCD studies had the depth and accuracy but not always the
resolution and certainly not the area coverage.
*Wal Sargent story...
Comparison of M31 CCD
Surveys
Basic Processing
Generally following the Valdes IRAF “pipeline”
but with some enhancements.
• Better flat-fielding techniques.
• Better determination of sky and scaling in
the stacking process (via scripts using
aperture photometry).
Details, and software, can be found at our web
site:
http://www.lowell.edu/~massey/lgsurvey
Photometry
For the purposes of photometry, we treat each
Mosaic camera as 8 separate instruments:
• PSF variations within a single chip modest
compared to chip-to-chip variations.
• Different DQE-wavelength dependence for
each chip means different color terms and
even different zero-points (despite flatfielding efforts).
U flat divided by I flat
Variations 30%
Photometry software
• It’s a factor of 40 times more work (8 chips x 5
ditherings) but at least when we’re done we have
1% photometry.
• We’ve developed a series of IRAF scripts and
FORTRAN programs that allow us to do the
photometry “automatically”, chip-by-chip, ditherby-dither.
• All of this is freely available from our web site:
http://www.lowell.edu/~massey/lgsurvey
How we’ve solved the calibration
problem
Lowell’s dark-sky site at Anderson Mesa
External Calibration using
Lowell ’s 1.2-m Hall Telescope
• Can use only the most pristine, photometric
nights.
• Select the best calibrated Landolt standards
covering a complete range of colors
– Investigate gravity effects on the U-band filter
U solution always squirrelly near
U-B=0.
U-B
It’s a matter of some gravity....
Progress Report---How are We
Doing?
• All images for M31 (10 fields), M33 (3 fields),
NGC 6822, IC10, WLM, Phoenix, Sextans A, and
Sextans B are now released, and sitting in the
NOAO “NSA” archive, as well as our own
dedicated ftp site (which makes bulk downloads
easier).
• Poor weather in early September prevented us
from completing the project: still need IC1613 and
the Pegasus dwarf, plus repeat of poor seeing
frames.
• Calibration in progress and catalog should be
complete on schedule, release Jan 2004.
Did We Achieve our 1.0” seeing
goal?
• Not really...
1.3”
0.76”
1.3”
0.76”
Poor seeing matters!
• To redo the images with seeing >= 1.3”
would require only a few additional nights.
Sadly...
We’ve been told that our time has run out, and
we aren’t eligible for additional time via the
survey TAC.
So, we’ve made our best case to the standard
TAC and we’ll see what happens.
(Wal Sargent Cautionary Tale)
M31 in
10 fields
M31 in
10 fields
M31
Fields
2 +3
M33North
M33-Center
NGC
6822
Phoenix
WLM
What’s Next?
• Spectroscopy!
M31
N206 in
M31
ob78-231
HST/ FUV ob78-231
Bianchi, Hutchings, Massey (1996, AJ, 111, 2303)
To take high S/N optical spectra
at B=19 requires a really big
telescope...
The 6.5-m
MMT
Optical (blue) spectrum ob78-231
Spectrum in
collaboration
with Kathy Eastwood
OB78-231 at H
Spectrum in
collaboration
with Kathy Eastwood
Meanwhile, these data are
already being used...
• Ben Williams’ PhD thesis (Univ Washington)
– Williams MNRAS 340, 143 based up a “bootstrap”
calibration.
• Forms optical basis for identifying “super-soft”
Chandra counterparts (DiStefano et al., in prep)
• Images have been featured in
– APOD 27 Sept 2001
– Astronomy Magazine (“Sky Gem” feature), Dec ‘02
– Upcoming Mercury article on “super star clusters”
(Hunter, Elmegreen, Massey 2003, in press)
Real Science
...will come once the calibrated photometry is
complete this summer. (Catalog will be
released at the AAS meeting in Jan 2004).
Follow-up Work In Progress
We’ll be pushing our studies of stellar winds
to high metallicities (M31) using Cycle 12
time on HST (40 orbits + 80 parallels just
awarded)
We also hope to begin extending our studies
of the IMF to the more distant galaxies of
the Local Group using DEIMOS on KeckII.